The invention involves a process and a printing machine for determining registration errors caused by different speeds of a sheet conveyor belt and a sheet moved on such conveyor belt.
In the printing of sheets of paper or the like by printing machines, the correctly positioned printing of the printed image on the sheet is of considerable importance. This characteristic is identified by the term registration. In order to set the registration, in addition to the printed image, registration marks are used, by which deviations from correctly positioned print are determined and measured by the operator of the printing machine. In a further embodiment of this process, the registration is determined and calculated using sensors in the printing machine. To do this, the sensors detect the registration marks on the conveyor belt or the sheet and determine, using the position of the registration marks, whether the printing is being done without errors.
The process and devices of the state of the art detect and correct errors that occur due to mechanical shifts of the sheet on the conveyor belt or shifts of the conveyor belt. Furthermore, errors occur which are caused by the speed of the conveyor belt differing from the speed of the sheet on the conveyor belt. This effect comes from the fact that the conveyor belt does not run straight at all positions, especially in the print modules, but runs in a curved line due to the rollers pressing onto the conveyor belt. The speed on the surface of the sheet is higher than the speed on the surface of the conveyor belt. As a result, the surface of the sheet covers a longer distance over time on the curved positions than the surface of the conveyor belt.
The distances covered, however, according to which the image is applied to the sheet, are defined by a specific time that passes during the movement of the conveyor belt between a sensor signal and a print module. As a result, a shifted application of the printed image occurs in the print modules, in which the sheet that lies on the conveyor covers a longer distance over time because of the non-straight progression of the conveyor belt and changing sheet thicknesses.
The purpose of the invention is thus to determine the registration errors described above, and correcting for the errors that are determined. The invention is achieved by a process and by a printing machine wherein there is an application of at least a first registration mark onto the conveyor belt and at least a second registration mark onto the sheet, a detection of the first registration mark and the second registration mark, a calculation of the distance between the first registration mark and the second registration mark and a comparison of the distance to a target value to determine a registration error correction value.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.